/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.io.Serializable;
import java.util.Collection;
import java.util.List;
import java.util.RandomAccess;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
import java.lang.reflect.Constructor;

/**
 * Abstract base class for tasks that run within a {@link ForkJoinPool}.
 * A {@code ForkJoinTask} is a thread-like entity that is much
 * lighter weight than a normal thread.  Huge numbers of tasks and
 * subtasks may be hosted by a small number of actual threads in a
 * ForkJoinPool, at the price of some usage limitations.
 *
 * <p>A "main" {@code ForkJoinTask} begins execution when it is
 * explicitly submitted to a {@link ForkJoinPool}, or, if not already
 * engaged in a ForkJoin computation, commenced in the {@link
 * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or
 * related methods.  Once started, it will usually in turn start other
 * subtasks.  As indicated by the name of this class, many programs
 * using {@code ForkJoinTask} employ only methods {@link #fork} and
 * {@link #join}, or derivatives such as {@link
 * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
 * provides a number of other methods that can come into play in
 * advanced usages, as well as extension mechanics that allow support
 * of new forms of fork/join processing.
 *
 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
 * The efficiency of {@code ForkJoinTask}s stems from a set of
 * restrictions (that are only partially statically enforceable)
 * reflecting their main use as computational tasks calculating pure
 * functions or operating on purely isolated objects.  The primary
 * coordination mechanisms are {@link #fork}, that arranges
 * asynchronous execution, and {@link #join}, that doesn't proceed
 * until the task's result has been computed.  Computations should
 * ideally avoid {@code synchronized} methods or blocks, and should
 * minimize other blocking synchronization apart from joining other
 * tasks or using synchronizers such as Phasers that are advertised to
 * cooperate with fork/join scheduling. Subdividable tasks should also
 * not perform blocking I/O, and should ideally access variables that
 * are completely independent of those accessed by other running
 * tasks. These guidelines are loosely enforced by not permitting
 * checked exceptions such as {@code IOExceptions} to be
 * thrown. However, computations may still encounter unchecked
 * exceptions, that are rethrown to callers attempting to join
 * them. These exceptions may additionally include {@link
 * RejectedExecutionException} stemming from internal resource
 * exhaustion, such as failure to allocate internal task
 * queues. Rethrown exceptions behave in the same way as regular
 * exceptions, but, when possible, contain stack traces (as displayed
 * for example using {@code ex.printStackTrace()}) of both the thread
 * that initiated the computation as well as the thread actually
 * encountering the exception; minimally only the latter.
 *
 * <p>It is possible to define and use ForkJoinTasks that may block,
 * but doing do requires three further considerations: (1) Completion
 * of few if any <em>other</em> tasks should be dependent on a task
 * that blocks on external synchronization or I/O. Event-style async
 * tasks that are never joined (for example, those subclassing {@link
 * CountedCompleter}) often fall into this category.  (2) To minimize
 * resource impact, tasks should be small; ideally performing only the
 * (possibly) blocking action. (3) Unless the {@link
 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
 * blocked tasks is known to be less than the pool's {@link
 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
 * enough threads will be available to ensure progress or good
 * performance.
 *
 * <p>The primary method for awaiting completion and extracting
 * results of a task is {@link #join}, but there are several variants:
 * The {@link Future#get} methods support interruptible and/or timed
 * waits for completion and report results using {@code Future}
 * conventions. Method {@link #invoke} is semantically
 * equivalent to {@code fork(); join()} but always attempts to begin
 * execution in the current thread. The "<em>quiet</em>" forms of
 * these methods do not extract results or report exceptions. These
 * may be useful when a set of tasks are being executed, and you need
 * to delay processing of results or exceptions until all complete.
 * Method {@code invokeAll} (available in multiple versions)
 * performs the most common form of parallel invocation: forking a set
 * of tasks and joining them all.
 *
 * <p>In the most typical usages, a fork-join pair act like a call
 * (fork) and return (join) from a parallel recursive function. As is
 * the case with other forms of recursive calls, returns (joins)
 * should be performed innermost-first. For example, {@code a.fork();
 * b.fork(); b.join(); a.join();} is likely to be substantially more
 * efficient than joining {@code a} before {@code b}.
 *
 * <p>The execution status of tasks may be queried at several levels
 * of detail: {@link #isDone} is true if a task completed in any way
 * (including the case where a task was cancelled without executing);
 * {@link #isCompletedNormally} is true if a task completed without
 * cancellation or encountering an exception; {@link #isCancelled} is
 * true if the task was cancelled (in which case {@link #getException}
 * returns a {@link java.util.concurrent.CancellationException}); and
 * {@link #isCompletedAbnormally} is true if a task was either
 * cancelled or encountered an exception, in which case {@link
 * #getException} will return either the encountered exception or
 * {@link java.util.concurrent.CancellationException}.
 *
 * <p>The ForkJoinTask class is not usually directly subclassed.
 * Instead, you subclass one of the abstract classes that support a
 * particular style of fork/join processing, typically {@link
 * RecursiveAction} for most computations that do not return results,
 * {@link RecursiveTask} for those that do, and {@link
 * CountedCompleter} for those in which completed actions trigger
 * other actions.  Normally, a concrete ForkJoinTask subclass declares
 * fields comprising its parameters, established in a constructor, and
 * then defines a {@code compute} method that somehow uses the control
 * methods supplied by this base class.
 *
 * <p>Method {@link #join} and its variants are appropriate for use
 * only when completion dependencies are acyclic; that is, the
 * parallel computation can be described as a directed acyclic graph
 * (DAG). Otherwise, executions may encounter a form of deadlock as
 * tasks cyclically wait for each other.  However, this framework
 * supports other methods and techniques (for example the use of
 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
 * may be of use in constructing custom subclasses for problems that
 * are not statically structured as DAGs. To support such usages, a
 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
 * value using {@link #setForkJoinTaskTag} or {@link
 * #compareAndSetForkJoinTaskTag} and checked using {@link
 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
 * these {@code protected} methods or tags for any purpose, but they
 * may be of use in the construction of specialized subclasses.  For
 * example, parallel graph traversals can use the supplied methods to
 * avoid revisiting nodes/tasks that have already been processed.
 * (Method names for tagging are bulky in part to encourage definition
 * of methods that reflect their usage patterns.)
 *
 * <p>Most base support methods are {@code final}, to prevent
 * overriding of implementations that are intrinsically tied to the
 * underlying lightweight task scheduling framework.  Developers
 * creating new basic styles of fork/join processing should minimally
 * implement {@code protected} methods {@link #exec}, {@link
 * #setRawResult}, and {@link #getRawResult}, while also introducing
 * an abstract computational method that can be implemented in its
 * subclasses, possibly relying on other {@code protected} methods
 * provided by this class.
 *
 * <p>ForkJoinTasks should perform relatively small amounts of
 * computation. Large tasks should be split into smaller subtasks,
 * usually via recursive decomposition. As a very rough rule of thumb,
 * a task should perform more than 100 and less than 10000 basic
 * computational steps, and should avoid indefinite looping. If tasks
 * are too big, then parallelism cannot improve throughput. If too
 * small, then memory and internal task maintenance overhead may
 * overwhelm processing.
 *
 * <p>This class provides {@code adapt} methods for {@link Runnable}
 * and {@link Callable}, that may be of use when mixing execution of
 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
 * of this form, consider using a pool constructed in <em>asyncMode</em>.
 *
 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be
 * used in extensions such as remote execution frameworks. It is
 * sensible to serialize tasks only before or after, but not during,
 * execution. Serialization is not relied on during execution itself.
 *
 * @author Doug Lea
 * @since 1.7
 */
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {

    /*
     * See the internal documentation of class ForkJoinPool for a
     * general implementation overview.  ForkJoinTasks are mainly
     * responsible for maintaining their "status" field amidst relays
     * to methods in ForkJoinWorkerThread and ForkJoinPool.
     *
     * The methods of this class are more-or-less layered into
     * (1) basic status maintenance
     * (2) execution and awaiting completion
     * (3) user-level methods that additionally report results.
     * This is sometimes hard to see because this file orders exported
     * methods in a way that flows well in javadocs.
     */

    /*
     * The status field holds run control status bits packed into a
     * single int to minimize footprint and to ensure atomicity (via
     * CAS).  Status is initially zero, and takes on nonnegative
     * values until completed, upon which status (anded with
     * DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks
     * undergoing blocking waits by other threads have the SIGNAL bit
     * set.  Completion of a stolen task with SIGNAL set awakens any
     * waiters via notifyAll. Even though suboptimal for some
     * purposes, we use basic builtin wait/notify to take advantage of
     * "monitor inflation" in JVMs that we would otherwise need to
     * emulate to avoid adding further per-task bookkeeping overhead.
     * We want these monitors to be "fat", i.e., not use biasing or
     * thin-lock techniques, so use some odd coding idioms that tend
     * to avoid them, mainly by arranging that every synchronized
     * block performs a wait, notifyAll or both.
     *
     * These control bits occupy only (some of) the upper half (16
     * bits) of status field. The lower bits are used for user-defined
     * tags.
     */

  /**
   * The run status of this task
   */
  volatile int status; // accessed directly by pool and workers
  static final int DONE_MASK = 0xf0000000;  // mask out non-completion bits
  static final int NORMAL = 0xf0000000;  // must be negative
  static final int CANCELLED = 0xc0000000;  // must be < NORMAL
  static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
  static final int SIGNAL = 0x00010000;  // must be >= 1 << 16
  static final int SMASK = 0x0000ffff;  // short bits for tags

  /**
   * Marks completion and wakes up threads waiting to join this
   * task.
   *
   * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
   * @return completion status on exit
   */
  private int setCompletion(int completion) {
    for (int s; ; ) {
      if ((s = status) < 0) {
        return s;
      }
      if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
        if ((s >>> 16) != 0) {
          synchronized (this) {
            notifyAll();
          }
        }
        return completion;
      }
    }
  }

  /**
   * Primary execution method for stolen tasks. Unless done, calls
   * exec and records status if completed, but doesn't wait for
   * completion otherwise.
   *
   * @return status on exit from this method
   */
  final int doExec() {
    int s;
    boolean completed;
    if ((s = status) >= 0) {
      try {
        completed = exec();
      } catch (Throwable rex) {
        return setExceptionalCompletion(rex);
      }
      if (completed) {
        s = setCompletion(NORMAL);
      }
    }
    return s;
  }

  /**
   * If not done, sets SIGNAL status and performs Object.wait(timeout).
   * This task may or may not be done on exit. Ignores interrupts.
   *
   * @param timeout using Object.wait conventions.
   */
  final void internalWait(long timeout) {
    int s;
    if ((s = status) >= 0 && // force completer to issue notify
        U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
      synchronized (this) {
        if (status >= 0) {
          try {
            wait(timeout);
          } catch (InterruptedException ie) {
          }
        } else {
          notifyAll();
        }
      }
    }
  }

  /**
   * Blocks a non-worker-thread until completion.
   *
   * @return status upon completion
   */
  private int externalAwaitDone() {
    int s = ((this instanceof CountedCompleter) ? // try helping
        ForkJoinPool.common.externalHelpComplete(
            (CountedCompleter<?>) this, 0) :
        ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0);
    if (s >= 0 && (s = status) >= 0) {
      boolean interrupted = false;
      do {
        if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
          synchronized (this) {
            if (status >= 0) {
              try {
                wait(0L);
              } catch (InterruptedException ie) {
                interrupted = true;
              }
            } else {
              notifyAll();
            }
          }
        }
      } while ((s = status) >= 0);
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
    return s;
  }

  /**
   * Blocks a non-worker-thread until completion or interruption.
   */
  private int externalInterruptibleAwaitDone() throws InterruptedException {
    int s;
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    if ((s = status) >= 0 &&
        (s = ((this instanceof CountedCompleter) ?
            ForkJoinPool.common.externalHelpComplete(
                (CountedCompleter<?>) this, 0) :
            ForkJoinPool.common.tryExternalUnpush(this) ? doExec() :
                0)) >= 0) {
      while ((s = status) >= 0) {
        if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
          synchronized (this) {
            if (status >= 0) {
              wait(0L);
            } else {
              notifyAll();
            }
          }
        }
      }
    }
    return s;
  }

  /**
   * Implementation for join, get, quietlyJoin. Directly handles
   * only cases of already-completed, external wait, and
   * unfork+exec.  Others are relayed to ForkJoinPool.awaitJoin.
   *
   * @return status upon completion
   */
  private int doJoin() {
    int s;
    Thread t;
    ForkJoinWorkerThread wt;
    ForkJoinPool.WorkQueue w;
    return (s = status) < 0 ? s :
        ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
            (w = (wt = (ForkJoinWorkerThread) t).workQueue).
                tryUnpush(this) && (s = doExec()) < 0 ? s :
                wt.pool.awaitJoin(w, this, 0L) :
            externalAwaitDone();
  }

  /**
   * Implementation for invoke, quietlyInvoke.
   *
   * @return status upon completion
   */
  private int doInvoke() {
    int s;
    Thread t;
    ForkJoinWorkerThread wt;
    return (s = doExec()) < 0 ? s :
        ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
            (wt = (ForkJoinWorkerThread) t).pool.
                awaitJoin(wt.workQueue, this, 0L) :
            externalAwaitDone();
  }

  // Exception table support

  /**
   * Table of exceptions thrown by tasks, to enable reporting by
   * callers. Because exceptions are rare, we don't directly keep
   * them with task objects, but instead use a weak ref table.  Note
   * that cancellation exceptions don't appear in the table, but are
   * instead recorded as status values.
   *
   * Note: These statics are initialized below in static block.
   */
  private static final ExceptionNode[] exceptionTable;
  private static final ReentrantLock exceptionTableLock;
  private static final ReferenceQueue<Object> exceptionTableRefQueue;

  /**
   * Fixed capacity for exceptionTable.
   */
  private static final int EXCEPTION_MAP_CAPACITY = 32;

  /**
   * Key-value nodes for exception table.  The chained hash table
   * uses identity comparisons, full locking, and weak references
   * for keys. The table has a fixed capacity because it only
   * maintains task exceptions long enough for joiners to access
   * them, so should never become very large for sustained
   * periods. However, since we do not know when the last joiner
   * completes, we must use weak references and expunge them. We do
   * so on each operation (hence full locking). Also, some thread in
   * any ForkJoinPool will call helpExpungeStaleExceptions when its
   * pool becomes isQuiescent.
   */
  static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {

    final Throwable ex;
    ExceptionNode next;
    final long thrower;  // use id not ref to avoid weak cycles
    final int hashCode;  // store task hashCode before weak ref disappears

    ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
      super(task, exceptionTableRefQueue);
      this.ex = ex;
      this.next = next;
      this.thrower = Thread.currentThread().getId();
      this.hashCode = System.identityHashCode(task);
    }
  }

  /**
   * Records exception and sets status.
   *
   * @return status on exit
   */
  final int recordExceptionalCompletion(Throwable ex) {
    int s;
    if ((s = status) >= 0) {
      int h = System.identityHashCode(this);
      final ReentrantLock lock = exceptionTableLock;
      lock.lock();
      try {
        expungeStaleExceptions();
        ExceptionNode[] t = exceptionTable;
        int i = h & (t.length - 1);
        for (ExceptionNode e = t[i]; ; e = e.next) {
          if (e == null) {
            t[i] = new ExceptionNode(this, ex, t[i]);
            break;
          }
          if (e.get() == this) // already present
          {
            break;
          }
        }
      } finally {
        lock.unlock();
      }
      s = setCompletion(EXCEPTIONAL);
    }
    return s;
  }

  /**
   * Records exception and possibly propagates.
   *
   * @return status on exit
   */
  private int setExceptionalCompletion(Throwable ex) {
    int s = recordExceptionalCompletion(ex);
    if ((s & DONE_MASK) == EXCEPTIONAL) {
      internalPropagateException(ex);
    }
    return s;
  }

  /**
   * Hook for exception propagation support for tasks with completers.
   */
  void internalPropagateException(Throwable ex) {
  }

  /**
   * Cancels, ignoring any exceptions thrown by cancel. Used during
   * worker and pool shutdown. Cancel is spec'ed not to throw any
   * exceptions, but if it does anyway, we have no recourse during
   * shutdown, so guard against this case.
   */
  static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
    if (t != null && t.status >= 0) {
      try {
        t.cancel(false);
      } catch (Throwable ignore) {
      }
    }
  }

  /**
   * Removes exception node and clears status.
   */
  private void clearExceptionalCompletion() {
    int h = System.identityHashCode(this);
    final ReentrantLock lock = exceptionTableLock;
    lock.lock();
    try {
      ExceptionNode[] t = exceptionTable;
      int i = h & (t.length - 1);
      ExceptionNode e = t[i];
      ExceptionNode pred = null;
      while (e != null) {
        ExceptionNode next = e.next;
        if (e.get() == this) {
          if (pred == null) {
            t[i] = next;
          } else {
            pred.next = next;
          }
          break;
        }
        pred = e;
        e = next;
      }
      expungeStaleExceptions();
      status = 0;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns a rethrowable exception for the given task, if
   * available. To provide accurate stack traces, if the exception
   * was not thrown by the current thread, we try to create a new
   * exception of the same type as the one thrown, but with the
   * recorded exception as its cause. If there is no such
   * constructor, we instead try to use a no-arg constructor,
   * followed by initCause, to the same effect. If none of these
   * apply, or any fail due to other exceptions, we return the
   * recorded exception, which is still correct, although it may
   * contain a misleading stack trace.
   *
   * @return the exception, or null if none
   */
  private Throwable getThrowableException() {
    if ((status & DONE_MASK) != EXCEPTIONAL) {
      return null;
    }
    int h = System.identityHashCode(this);
    ExceptionNode e;
    final ReentrantLock lock = exceptionTableLock;
    lock.lock();
    try {
      expungeStaleExceptions();
      ExceptionNode[] t = exceptionTable;
      e = t[h & (t.length - 1)];
      while (e != null && e.get() != this) {
        e = e.next;
      }
    } finally {
      lock.unlock();
    }
    Throwable ex;
    if (e == null || (ex = e.ex) == null) {
      return null;
    }
    if (e.thrower != Thread.currentThread().getId()) {
      Class<? extends Throwable> ec = ex.getClass();
      try {
        Constructor<?> noArgCtor = null;
        Constructor<?>[] cs = ec.getConstructors();// public ctors only
        for (int i = 0; i < cs.length; ++i) {
          Constructor<?> c = cs[i];
          Class<?>[] ps = c.getParameterTypes();
          if (ps.length == 0) {
            noArgCtor = c;
          } else if (ps.length == 1 && ps[0] == Throwable.class) {
            Throwable wx = (Throwable) c.newInstance(ex);
            return (wx == null) ? ex : wx;
          }
        }
        if (noArgCtor != null) {
          Throwable wx = (Throwable) (noArgCtor.newInstance());
          if (wx != null) {
            wx.initCause(ex);
            return wx;
          }
        }
      } catch (Exception ignore) {
      }
    }
    return ex;
  }

  /**
   * Poll stale refs and remove them. Call only while holding lock.
   */
  private static void expungeStaleExceptions() {
    for (Object x; (x = exceptionTableRefQueue.poll()) != null; ) {
      if (x instanceof ExceptionNode) {
        int hashCode = ((ExceptionNode) x).hashCode;
        ExceptionNode[] t = exceptionTable;
        int i = hashCode & (t.length - 1);
        ExceptionNode e = t[i];
        ExceptionNode pred = null;
        while (e != null) {
          ExceptionNode next = e.next;
          if (e == x) {
            if (pred == null) {
              t[i] = next;
            } else {
              pred.next = next;
            }
            break;
          }
          pred = e;
          e = next;
        }
      }
    }
  }

  /**
   * If lock is available, poll stale refs and remove them.
   * Called from ForkJoinPool when pools become quiescent.
   */
  static final void helpExpungeStaleExceptions() {
    final ReentrantLock lock = exceptionTableLock;
    if (lock.tryLock()) {
      try {
        expungeStaleExceptions();
      } finally {
        lock.unlock();
      }
    }
  }

  /**
   * A version of "sneaky throw" to relay exceptions
   */
  static void rethrow(Throwable ex) {
    if (ex != null) {
      ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
    }
  }

  /**
   * The sneaky part of sneaky throw, relying on generics
   * limitations to evade compiler complaints about rethrowing
   * unchecked exceptions
   */
  @SuppressWarnings("unchecked")
  static <T extends Throwable>
  void uncheckedThrow(Throwable t) throws T {
    throw (T) t; // rely on vacuous cast
  }

  /**
   * Throws exception, if any, associated with the given status.
   */
  private void reportException(int s) {
    if (s == CANCELLED) {
      throw new CancellationException();
    }
    if (s == EXCEPTIONAL) {
      rethrow(getThrowableException());
    }
  }

  // public methods

  /**
   * Arranges to asynchronously execute this task in the pool the
   * current task is running in, if applicable, or using the {@link
   * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While
   * it is not necessarily enforced, it is a usage error to fork a
   * task more than once unless it has completed and been
   * reinitialized.  Subsequent modifications to the state of this
   * task or any data it operates on are not necessarily
   * consistently observable by any thread other than the one
   * executing it unless preceded by a call to {@link #join} or
   * related methods, or a call to {@link #isDone} returning {@code
   * true}.
   *
   * @return {@code this}, to simplify usage
   */
  public final ForkJoinTask<V> fork() {
    Thread t;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
      ((ForkJoinWorkerThread) t).workQueue.push(this);
    } else {
      ForkJoinPool.common.externalPush(this);
    }
    return this;
  }

  /**
   * Returns the result of the computation when it {@link #isDone is
   * done}.  This method differs from {@link #get()} in that
   * abnormal completion results in {@code RuntimeException} or
   * {@code Error}, not {@code ExecutionException}, and that
   * interrupts of the calling thread do <em>not</em> cause the
   * method to abruptly return by throwing {@code
   * InterruptedException}.
   *
   * @return the computed result
   */
  public final V join() {
    int s;
    if ((s = doJoin() & DONE_MASK) != NORMAL) {
      reportException(s);
    }
    return getRawResult();
  }

  /**
   * Commences performing this task, awaits its completion if
   * necessary, and returns its result, or throws an (unchecked)
   * {@code RuntimeException} or {@code Error} if the underlying
   * computation did so.
   *
   * @return the computed result
   */
  public final V invoke() {
    int s;
    if ((s = doInvoke() & DONE_MASK) != NORMAL) {
      reportException(s);
    }
    return getRawResult();
  }

  /**
   * Forks the given tasks, returning when {@code isDone} holds for
   * each task or an (unchecked) exception is encountered, in which
   * case the exception is rethrown. If more than one task
   * encounters an exception, then this method throws any one of
   * these exceptions. If any task encounters an exception, the
   * other may be cancelled. However, the execution status of
   * individual tasks is not guaranteed upon exceptional return. The
   * status of each task may be obtained using {@link
   * #getException()} and related methods to check if they have been
   * cancelled, completed normally or exceptionally, or left
   * unprocessed.
   *
   * @param t1 the first task
   * @param t2 the second task
   * @throws NullPointerException if any task is null
   */
  public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
    int s1, s2;
    t2.fork();
    if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL) {
      t1.reportException(s1);
    }
    if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL) {
      t2.reportException(s2);
    }
  }

  /**
   * Forks the given tasks, returning when {@code isDone} holds for
   * each task or an (unchecked) exception is encountered, in which
   * case the exception is rethrown. If more than one task
   * encounters an exception, then this method throws any one of
   * these exceptions. If any task encounters an exception, others
   * may be cancelled. However, the execution status of individual
   * tasks is not guaranteed upon exceptional return. The status of
   * each task may be obtained using {@link #getException()} and
   * related methods to check if they have been cancelled, completed
   * normally or exceptionally, or left unprocessed.
   *
   * @param tasks the tasks
   * @throws NullPointerException if any task is null
   */
  public static void invokeAll(ForkJoinTask<?>... tasks) {
    Throwable ex = null;
    int last = tasks.length - 1;
    for (int i = last; i >= 0; --i) {
      ForkJoinTask<?> t = tasks[i];
      if (t == null) {
        if (ex == null) {
          ex = new NullPointerException();
        }
      } else if (i != 0) {
        t.fork();
      } else if (t.doInvoke() < NORMAL && ex == null) {
        ex = t.getException();
      }
    }
    for (int i = 1; i <= last; ++i) {
      ForkJoinTask<?> t = tasks[i];
      if (t != null) {
        if (ex != null) {
          t.cancel(false);
        } else if (t.doJoin() < NORMAL) {
          ex = t.getException();
        }
      }
    }
    if (ex != null) {
      rethrow(ex);
    }
  }

  /**
   * Forks all tasks in the specified collection, returning when
   * {@code isDone} holds for each task or an (unchecked) exception
   * is encountered, in which case the exception is rethrown. If
   * more than one task encounters an exception, then this method
   * throws any one of these exceptions. If any task encounters an
   * exception, others may be cancelled. However, the execution
   * status of individual tasks is not guaranteed upon exceptional
   * return. The status of each task may be obtained using {@link
   * #getException()} and related methods to check if they have been
   * cancelled, completed normally or exceptionally, or left
   * unprocessed.
   *
   * @param tasks the collection of tasks
   * @param <T> the type of the values returned from the tasks
   * @return the tasks argument, to simplify usage
   * @throws NullPointerException if tasks or any element are null
   */
  public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
    if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
      invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
      return tasks;
    }
    @SuppressWarnings("unchecked")
    List<? extends ForkJoinTask<?>> ts =
        (List<? extends ForkJoinTask<?>>) tasks;
    Throwable ex = null;
    int last = ts.size() - 1;
    for (int i = last; i >= 0; --i) {
      ForkJoinTask<?> t = ts.get(i);
      if (t == null) {
        if (ex == null) {
          ex = new NullPointerException();
        }
      } else if (i != 0) {
        t.fork();
      } else if (t.doInvoke() < NORMAL && ex == null) {
        ex = t.getException();
      }
    }
    for (int i = 1; i <= last; ++i) {
      ForkJoinTask<?> t = ts.get(i);
      if (t != null) {
        if (ex != null) {
          t.cancel(false);
        } else if (t.doJoin() < NORMAL) {
          ex = t.getException();
        }
      }
    }
    if (ex != null) {
      rethrow(ex);
    }
    return tasks;
  }

  /**
   * Attempts to cancel execution of this task. This attempt will
   * fail if the task has already completed or could not be
   * cancelled for some other reason. If successful, and this task
   * has not started when {@code cancel} is called, execution of
   * this task is suppressed. After this method returns
   * successfully, unless there is an intervening call to {@link
   * #reinitialize}, subsequent calls to {@link #isCancelled},
   * {@link #isDone}, and {@code cancel} will return {@code true}
   * and calls to {@link #join} and related methods will result in
   * {@code CancellationException}.
   *
   * <p>This method may be overridden in subclasses, but if so, must
   * still ensure that these properties hold. In particular, the
   * {@code cancel} method itself must not throw exceptions.
   *
   * <p>This method is designed to be invoked by <em>other</em>
   * tasks. To terminate the current task, you can just return or
   * throw an unchecked exception from its computation method, or
   * invoke {@link #completeExceptionally(Throwable)}.
   *
   * @param mayInterruptIfRunning this value has no effect in the default implementation because
   * interrupts are not used to control cancellation.
   * @return {@code true} if this task is now cancelled
   */
  public boolean cancel(boolean mayInterruptIfRunning) {
    return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
  }

  public final boolean isDone() {
    return status < 0;
  }

  public final boolean isCancelled() {
    return (status & DONE_MASK) == CANCELLED;
  }

  /**
   * Returns {@code true} if this task threw an exception or was cancelled.
   *
   * @return {@code true} if this task threw an exception or was cancelled
   */
  public final boolean isCompletedAbnormally() {
    return status < NORMAL;
  }

  /**
   * Returns {@code true} if this task completed without throwing an
   * exception and was not cancelled.
   *
   * @return {@code true} if this task completed without throwing an exception and was not cancelled
   */
  public final boolean isCompletedNormally() {
    return (status & DONE_MASK) == NORMAL;
  }

  /**
   * Returns the exception thrown by the base computation, or a
   * {@code CancellationException} if cancelled, or {@code null} if
   * none or if the method has not yet completed.
   *
   * @return the exception, or {@code null} if none
   */
  public final Throwable getException() {
    int s = status & DONE_MASK;
    return ((s >= NORMAL) ? null :
        (s == CANCELLED) ? new CancellationException() :
            getThrowableException());
  }

  /**
   * Completes this task abnormally, and if not already aborted or
   * cancelled, causes it to throw the given exception upon
   * {@code join} and related operations. This method may be used
   * to induce exceptions in asynchronous tasks, or to force
   * completion of tasks that would not otherwise complete.  Its use
   * in other situations is discouraged.  This method is
   * overridable, but overridden versions must invoke {@code super}
   * implementation to maintain guarantees.
   *
   * @param ex the exception to throw. If this exception is not a {@code RuntimeException} or {@code
   * Error}, the actual exception thrown will be a {@code RuntimeException} with cause {@code ex}.
   */
  public void completeExceptionally(Throwable ex) {
    setExceptionalCompletion((ex instanceof RuntimeException) ||
        (ex instanceof Error) ? ex :
        new RuntimeException(ex));
  }

  /**
   * Completes this task, and if not already aborted or cancelled,
   * returning the given value as the result of subsequent
   * invocations of {@code join} and related operations. This method
   * may be used to provide results for asynchronous tasks, or to
   * provide alternative handling for tasks that would not otherwise
   * complete normally. Its use in other situations is
   * discouraged. This method is overridable, but overridden
   * versions must invoke {@code super} implementation to maintain
   * guarantees.
   *
   * @param value the result value for this task
   */
  public void complete(V value) {
    try {
      setRawResult(value);
    } catch (Throwable rex) {
      setExceptionalCompletion(rex);
      return;
    }
    setCompletion(NORMAL);
  }

  /**
   * Completes this task normally without setting a value. The most
   * recent value established by {@link #setRawResult} (or {@code
   * null} by default) will be returned as the result of subsequent
   * invocations of {@code join} and related operations.
   *
   * @since 1.8
   */
  public final void quietlyComplete() {
    setCompletion(NORMAL);
  }

  /**
   * Waits if necessary for the computation to complete, and then
   * retrieves its result.
   *
   * @return the computed result
   * @throws CancellationException if the computation was cancelled
   * @throws ExecutionException if the computation threw an exception
   * @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
   * interrupted while waiting
   */
  public final V get() throws InterruptedException, ExecutionException {
    int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
        doJoin() : externalInterruptibleAwaitDone();
    Throwable ex;
    if ((s &= DONE_MASK) == CANCELLED) {
      throw new CancellationException();
    }
    if (s == EXCEPTIONAL && (ex = getThrowableException()) != null) {
      throw new ExecutionException(ex);
    }
    return getRawResult();
  }

  /**
   * Waits if necessary for at most the given time for the computation
   * to complete, and then retrieves its result, if available.
   *
   * @param timeout the maximum time to wait
   * @param unit the time unit of the timeout argument
   * @return the computed result
   * @throws CancellationException if the computation was cancelled
   * @throws ExecutionException if the computation threw an exception
   * @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
   * interrupted while waiting
   * @throws TimeoutException if the wait timed out
   */
  public final V get(long timeout, TimeUnit unit)
      throws InterruptedException, ExecutionException, TimeoutException {
    int s;
    long nanos = unit.toNanos(timeout);
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    if ((s = status) >= 0 && nanos > 0L) {
      long d = System.nanoTime() + nanos;
      long deadline = (d == 0L) ? 1L : d; // avoid 0
      Thread t = Thread.currentThread();
      if (t instanceof ForkJoinWorkerThread) {
        ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
        s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
      } else if ((s = ((this instanceof CountedCompleter) ?
          ForkJoinPool.common.externalHelpComplete(
              (CountedCompleter<?>) this, 0) :
          ForkJoinPool.common.tryExternalUnpush(this) ?
              doExec() : 0)) >= 0) {
        long ns, ms; // measure in nanosecs, but wait in millisecs
        while ((s = status) >= 0 &&
            (ns = deadline - System.nanoTime()) > 0L) {
          if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
              U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
            synchronized (this) {
              if (status >= 0) {
                wait(ms); // OK to throw InterruptedException
              } else {
                notifyAll();
              }
            }
          }
        }
      }
    }
    if (s >= 0) {
      s = status;
    }
    if ((s &= DONE_MASK) != NORMAL) {
      Throwable ex;
      if (s == CANCELLED) {
        throw new CancellationException();
      }
      if (s != EXCEPTIONAL) {
        throw new TimeoutException();
      }
      if ((ex = getThrowableException()) != null) {
        throw new ExecutionException(ex);
      }
    }
    return getRawResult();
  }

  /**
   * Joins this task, without returning its result or throwing its
   * exception. This method may be useful when processing
   * collections of tasks when some have been cancelled or otherwise
   * known to have aborted.
   */
  public final void quietlyJoin() {
    doJoin();
  }

  /**
   * Commences performing this task and awaits its completion if
   * necessary, without returning its result or throwing its
   * exception.
   */
  public final void quietlyInvoke() {
    doInvoke();
  }

  /**
   * Possibly executes tasks until the pool hosting the current task
   * {@link ForkJoinPool#isQuiescent is quiescent}. This method may
   * be of use in designs in which many tasks are forked, but none
   * are explicitly joined, instead executing them until all are
   * processed.
   */
  public static void helpQuiesce() {
    Thread t;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
      ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
      wt.pool.helpQuiescePool(wt.workQueue);
    } else {
      ForkJoinPool.quiesceCommonPool();
    }
  }

  /**
   * Resets the internal bookkeeping state of this task, allowing a
   * subsequent {@code fork}. This method allows repeated reuse of
   * this task, but only if reuse occurs when this task has either
   * never been forked, or has been forked, then completed and all
   * outstanding joins of this task have also completed. Effects
   * under any other usage conditions are not guaranteed.
   * This method may be useful when executing
   * pre-constructed trees of subtasks in loops.
   *
   * <p>Upon completion of this method, {@code isDone()} reports
   * {@code false}, and {@code getException()} reports {@code
   * null}. However, the value returned by {@code getRawResult} is
   * unaffected. To clear this value, you can invoke {@code
   * setRawResult(null)}.
   */
  public void reinitialize() {
    if ((status & DONE_MASK) == EXCEPTIONAL) {
      clearExceptionalCompletion();
    } else {
      status = 0;
    }
  }

  /**
   * Returns the pool hosting the current task execution, or null
   * if this task is executing outside of any ForkJoinPool.
   *
   * @return the pool, or {@code null} if none
   * @see #inForkJoinPool
   */
  public static ForkJoinPool getPool() {
    Thread t = Thread.currentThread();
    return (t instanceof ForkJoinWorkerThread) ?
        ((ForkJoinWorkerThread) t).pool : null;
  }

  /**
   * Returns {@code true} if the current thread is a {@link
   * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
   *
   * @return {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
   * ForkJoinPool computation, or {@code false} otherwise
   */
  public static boolean inForkJoinPool() {
    return Thread.currentThread() instanceof ForkJoinWorkerThread;
  }

  /**
   * Tries to unschedule this task for execution. This method will
   * typically (but is not guaranteed to) succeed if this task is
   * the most recently forked task by the current thread, and has
   * not commenced executing in another thread.  This method may be
   * useful when arranging alternative local processing of tasks
   * that could have been, but were not, stolen.
   *
   * @return {@code true} if unforked
   */
  public boolean tryUnfork() {
    Thread t;
    return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
        ((ForkJoinWorkerThread) t).workQueue.tryUnpush(this) :
        ForkJoinPool.common.tryExternalUnpush(this));
  }

  /**
   * Returns an estimate of the number of tasks that have been
   * forked by the current worker thread but not yet executed. This
   * value may be useful for heuristic decisions about whether to
   * fork other tasks.
   *
   * @return the number of tasks
   */
  public static int getQueuedTaskCount() {
    Thread t;
    ForkJoinPool.WorkQueue q;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
      q = ((ForkJoinWorkerThread) t).workQueue;
    } else {
      q = ForkJoinPool.commonSubmitterQueue();
    }
    return (q == null) ? 0 : q.queueSize();
  }

  /**
   * Returns an estimate of how many more locally queued tasks are
   * held by the current worker thread than there are other worker
   * threads that might steal them, or zero if this thread is not
   * operating in a ForkJoinPool. This value may be useful for
   * heuristic decisions about whether to fork other tasks. In many
   * usages of ForkJoinTasks, at steady state, each worker should
   * aim to maintain a small constant surplus (for example, 3) of
   * tasks, and to process computations locally if this threshold is
   * exceeded.
   *
   * @return the surplus number of tasks, which may be negative
   */
  public static int getSurplusQueuedTaskCount() {
    return ForkJoinPool.getSurplusQueuedTaskCount();
  }

  // Extension methods

  /**
   * Returns the result that would be returned by {@link #join}, even
   * if this task completed abnormally, or {@code null} if this task
   * is not known to have been completed.  This method is designed
   * to aid debugging, as well as to support extensions. Its use in
   * any other context is discouraged.
   *
   * @return the result, or {@code null} if not completed
   */
  public abstract V getRawResult();

  /**
   * Forces the given value to be returned as a result.  This method
   * is designed to support extensions, and should not in general be
   * called otherwise.
   *
   * @param value the value
   */
  protected abstract void setRawResult(V value);

  /**
   * Immediately performs the base action of this task and returns
   * true if, upon return from this method, this task is guaranteed
   * to have completed normally. This method may return false
   * otherwise, to indicate that this task is not necessarily
   * complete (or is not known to be complete), for example in
   * asynchronous actions that require explicit invocations of
   * completion methods. This method may also throw an (unchecked)
   * exception to indicate abnormal exit. This method is designed to
   * support extensions, and should not in general be called
   * otherwise.
   *
   * @return {@code true} if this task is known to have completed normally
   */
  protected abstract boolean exec();

  /**
   * Returns, but does not unschedule or execute, a task queued by
   * the current thread but not yet executed, if one is immediately
   * available. There is no guarantee that this task will actually
   * be polled or executed next. Conversely, this method may return
   * null even if a task exists but cannot be accessed without
   * contention with other threads.  This method is designed
   * primarily to support extensions, and is unlikely to be useful
   * otherwise.
   *
   * @return the next task, or {@code null} if none are available
   */
  protected static ForkJoinTask<?> peekNextLocalTask() {
    Thread t;
    ForkJoinPool.WorkQueue q;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
      q = ((ForkJoinWorkerThread) t).workQueue;
    } else {
      q = ForkJoinPool.commonSubmitterQueue();
    }
    return (q == null) ? null : q.peek();
  }

  /**
   * Unschedules and returns, without executing, the next task
   * queued by the current thread but not yet executed, if the
   * current thread is operating in a ForkJoinPool.  This method is
   * designed primarily to support extensions, and is unlikely to be
   * useful otherwise.
   *
   * @return the next task, or {@code null} if none are available
   */
  protected static ForkJoinTask<?> pollNextLocalTask() {
    Thread t;
    return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
        ((ForkJoinWorkerThread) t).workQueue.nextLocalTask() :
        null;
  }

  /**
   * If the current thread is operating in a ForkJoinPool,
   * unschedules and returns, without executing, the next task
   * queued by the current thread but not yet executed, if one is
   * available, or if not available, a task that was forked by some
   * other thread, if available. Availability may be transient, so a
   * {@code null} result does not necessarily imply quiescence of
   * the pool this task is operating in.  This method is designed
   * primarily to support extensions, and is unlikely to be useful
   * otherwise.
   *
   * @return a task, or {@code null} if none are available
   */
  protected static ForkJoinTask<?> pollTask() {
    Thread t;
    ForkJoinWorkerThread wt;
    return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
        (wt = (ForkJoinWorkerThread) t).pool.nextTaskFor(wt.workQueue) :
        null;
  }

  // tag operations

  /**
   * Returns the tag for this task.
   *
   * @return the tag for this task
   * @since 1.8
   */
  public final short getForkJoinTaskTag() {
    return (short) status;
  }

  /**
   * Atomically sets the tag value for this task.
   *
   * @param tag the tag value
   * @return the previous value of the tag
   * @since 1.8
   */
  public final short setForkJoinTaskTag(short tag) {
    for (int s; ; ) {
      if (U.compareAndSwapInt(this, STATUS, s = status,
          (s & ~SMASK) | (tag & SMASK))) {
        return (short) s;
      }
    }
  }

  /**
   * Atomically conditionally sets the tag value for this task.
   * Among other applications, tags can be used as visit markers
   * in tasks operating on graphs, as in methods that check: {@code
   * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
   * before processing, otherwise exiting because the node has
   * already been visited.
   *
   * @param e the expected tag value
   * @param tag the new tag value
   * @return {@code true} if successful; i.e., the current value was equal to e and is now tag.
   * @since 1.8
   */
  public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
    for (int s; ; ) {
      if ((short) (s = status) != e) {
        return false;
      }
      if (U.compareAndSwapInt(this, STATUS, s,
          (s & ~SMASK) | (tag & SMASK))) {
        return true;
      }
    }
  }

  /**
   * Adaptor for Runnables. This implements RunnableFuture
   * to be compliant with AbstractExecutorService constraints
   * when used in ForkJoinPool.
   */
  static final class AdaptedRunnable<T> extends ForkJoinTask<T>
      implements RunnableFuture<T> {

    final Runnable runnable;
    T result;

    AdaptedRunnable(Runnable runnable, T result) {
      if (runnable == null) {
        throw new NullPointerException();
      }
      this.runnable = runnable;
      this.result = result; // OK to set this even before completion
    }

    public final T getRawResult() {
      return result;
    }

    public final void setRawResult(T v) {
      result = v;
    }

    public final boolean exec() {
      runnable.run();
      return true;
    }

    public final void run() {
      invoke();
    }

    private static final long serialVersionUID = 5232453952276885070L;
  }

  /**
   * Adaptor for Runnables without results
   */
  static final class AdaptedRunnableAction extends ForkJoinTask<Void>
      implements RunnableFuture<Void> {

    final Runnable runnable;

    AdaptedRunnableAction(Runnable runnable) {
      if (runnable == null) {
        throw new NullPointerException();
      }
      this.runnable = runnable;
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {
    }

    public final boolean exec() {
      runnable.run();
      return true;
    }

    public final void run() {
      invoke();
    }

    private static final long serialVersionUID = 5232453952276885070L;
  }

  /**
   * Adaptor for Runnables in which failure forces worker exception
   */
  static final class RunnableExecuteAction extends ForkJoinTask<Void> {

    final Runnable runnable;

    RunnableExecuteAction(Runnable runnable) {
      if (runnable == null) {
        throw new NullPointerException();
      }
      this.runnable = runnable;
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {
    }

    public final boolean exec() {
      runnable.run();
      return true;
    }

    void internalPropagateException(Throwable ex) {
      rethrow(ex); // rethrow outside exec() catches.
    }

    private static final long serialVersionUID = 5232453952276885070L;
  }

  /**
   * Adaptor for Callables
   */
  static final class AdaptedCallable<T> extends ForkJoinTask<T>
      implements RunnableFuture<T> {

    final Callable<? extends T> callable;
    T result;

    AdaptedCallable(Callable<? extends T> callable) {
      if (callable == null) {
        throw new NullPointerException();
      }
      this.callable = callable;
    }

    public final T getRawResult() {
      return result;
    }

    public final void setRawResult(T v) {
      result = v;
    }

    public final boolean exec() {
      try {
        result = callable.call();
        return true;
      } catch (Error err) {
        throw err;
      } catch (RuntimeException rex) {
        throw rex;
      } catch (Exception ex) {
        throw new RuntimeException(ex);
      }
    }

    public final void run() {
      invoke();
    }

    private static final long serialVersionUID = 2838392045355241008L;
  }

  /**
   * Returns a new {@code ForkJoinTask} that performs the {@code run}
   * method of the given {@code Runnable} as its action, and returns
   * a null result upon {@link #join}.
   *
   * @param runnable the runnable action
   * @return the task
   */
  public static ForkJoinTask<?> adapt(Runnable runnable) {
    return new AdaptedRunnableAction(runnable);
  }

  /**
   * Returns a new {@code ForkJoinTask} that performs the {@code run}
   * method of the given {@code Runnable} as its action, and returns
   * the given result upon {@link #join}.
   *
   * @param runnable the runnable action
   * @param result the result upon completion
   * @param <T> the type of the result
   * @return the task
   */
  public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
    return new AdaptedRunnable<T>(runnable, result);
  }

  /**
   * Returns a new {@code ForkJoinTask} that performs the {@code call}
   * method of the given {@code Callable} as its action, and returns
   * its result upon {@link #join}, translating any checked exceptions
   * encountered into {@code RuntimeException}.
   *
   * @param callable the callable action
   * @param <T> the type of the callable's result
   * @return the task
   */
  public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
    return new AdaptedCallable<T>(callable);
  }

  // Serialization support

  private static final long serialVersionUID = -7721805057305804111L;

  /**
   * Saves this task to a stream (that is, serializes it).
   *
   * @param s the stream
   * @throws java.io.IOException if an I/O error occurs
   * @serialData the current run status and the exception thrown during execution, or {@code null}
   * if none
   */
  private void writeObject(java.io.ObjectOutputStream s)
      throws java.io.IOException {
    s.defaultWriteObject();
    s.writeObject(getException());
  }

  /**
   * Reconstitutes this task from a stream (that is, deserializes it).
   *
   * @param s the stream
   * @throws ClassNotFoundException if the class of a serialized object could not be found
   * @throws java.io.IOException if an I/O error occurs
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    s.defaultReadObject();
    Object ex = s.readObject();
    if (ex != null) {
      setExceptionalCompletion((Throwable) ex);
    }
  }

  // Unsafe mechanics
  private static final sun.misc.Unsafe U;
  private static final long STATUS;

  static {
    exceptionTableLock = new ReentrantLock();
    exceptionTableRefQueue = new ReferenceQueue<Object>();
    exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
    try {
      U = sun.misc.Unsafe.getUnsafe();
      Class<?> k = ForkJoinTask.class;
      STATUS = U.objectFieldOffset
          (k.getDeclaredField("status"));
    } catch (Exception e) {
      throw new Error(e);
    }
  }

}
